The present invention relates to power supplies. More specifically, the present invention relates to a wide range single-phase/polyphase switching power supply particularly well suited for use with electronic electricity meters.
One example of a prior art power supply used with electronic electricity meters is schematically shown in FIGS. 1A and B, herein, labeled prior art. Referring to FIGS. 1A and B, a power supply 10 comprises a full wave bridge circuit 12, which operates on only one phase, and is utilized with both single phase and polyphase meters. The rectified output of bridge circuit 12 is connected to a preregulator circuit 16 where the primary input voltage from bridge circuit 12 is limited to 300 volts, the controller is also cut out at 300 volts. The output of preregulator circuit 16 is presented to a filter circuit 18 comprising a capacitor for supply stability. A bootstrap circuit 20 is connected to a controller circuit 22 for startup. Bootstrap circuit 20 comprises a FET circuit, which starts at a low voltage with low power dissipation at high input voltages.
An output of controller circuit 22 drives the gate of a switching FET 24, at a preset frequency and variable duty cycle. The aforementioned filtered voltage signal and the drain of switching FET 24 are connected across a first winding on the primary side of a flyback transformer 26. The source of switching FET 24 is connected through a resistor to ground. The source of switching FET 24 is also connected to a resistor and capacitor filter/current sense circuit 28 with the output thereof connected to the current sense input of controller circuit 22. A second winding on the primary side of the flyback transformer 26 is connected across Vcc, the supply voltage of the controller circuit 22, and the compensation input of the controller circuit 22 to provide feedback. It will be appreciated that providing feedback directly from the transformer winding in conjunction with the voltage cutout, results in poor rejection of the 120 Hz ripple in the voltage signal, which necessitates regulation of the voltage signal. In accordance with this requirement, the windings on the secondary side of the flyback transformer 26 are connected to positive and negative supply filter and line regulators 30 and 32, respectively.